> On the off chance that my earlier reference to the half-life of U-235 was mis-taken
> as cheap hystericizing or dopey moralizing, let me elaborate and sharpen the point.
I missed it. Had I seen it, I would have considered it hysterical. No mistake.
U-235 is not the big issue, precisely because its half-life is in the millions of years. Less than a millionth of it decays in a year. Besides, it is alpha decay. Almost anything stops alpha particles -- a few cm of air, a sheet of paper, ... and they don't do much damage anyway.
Things with a really short half-life are very dangerous, but not for long. For a one-day half-life, the radiation is below 1% of the original level in a week, below a billionth in a month.
Some of the products of nuclear fission with intermediate half-lives, though, are remarkably nasty. Take Strontium 90, half-life about 30 years. It does beta decay, and the stuff it decays to does the same with a half-life of a few days. Two beta particles for the price of one. Worse, the stuff acts chemically like calcium and can replace it in milk, bones, ... Yet it takes 30 years to drop to half the radioactivity. Ouch! Many of us are probably carrying a bit of that from the old atmospheric weapons tests.
Even that is only beta decay, though. Fission produces dozens of products. I'm no physicist, so I do not know if there is one with a medium half-life and gamma decay. That would be most dangerous of the lot.